You might be able to write an entire list like this just on the ill-fated second season of Friday Night Lights. The writer's strike-shortened second season of FNL was easily the worst, and the creative team decided they would start fresh in season three. This meant abandoning some of the show's more infamous storylines and jettisoning characters like the delinquent trying to get his life back on track.
Santiago (Benny Ciaramello), we hardly knew ye, as you joined us and left us all in the course of one short season. We never really cared for your storyline, but you live on in pretty much every list like this one.